Coarsely chop the onion; then put it in the food processor and pulse until onion is finely chopped. Add the seasonings and pulse a few more times until the seasonings are well blended with onion.
Put the ground beef into a bowl, add the onion-spice mixture, and use your clean hands to mix the spices into the meat.
Then use a TBSP-sized measuring spoon to scoop out the meat and form into meatballs. (Should make about 20 meatballs)
Brush a stove-top grill pan or frying pan with olive oil, then heat the pan over medium high heat and cook the meatballs, turning often, until they are well browned and completely cooked through (10-12 mins)
Make a gyro by using butter lettuce, meatballs, tomatoes, tzatziki and any other ingredients.